Challenges and prospects for multi-chip microlens imprints on front-side illuminated SPAD imagers.

The overall sensitivity of frontside-illuminated, silicon single-photon avalanche diode (SPAD) arrays has often suffered from fill factor limitations. The fill factor loss can however be recovered by employing microlenses, whereby the challenges specific to SPAD arrays are represented by large pixel pitch (> 10 µm), low native fill factor (as low as ∼10%), and large size (up to 10 mm). In this work we report on the implementation of refractive microlenses by means of photoresist masters, used to fabricate molds for imprints of UV curable hybrid polymers deposited on SPAD arrays. Replications were successfully carried out for the first time, to the best of our knowledge, at wafer reticle level on different designs in the same technology and on single large SPAD arrays with very thin residual layers (∼10 µm), as needed for better efficiency at higher numerical aperture (NA > 0.25). In general, concentration factors within 15-20% of the simulation results were obtained for the smaller arrays (32×32 and 512×1), achieving for example an effective fill factor of 75.6-83.2% for a 28.5 µm pixel pitch with a native fill factor of 28%. A concentration factor up to 4.2 was measured on large 512×512 arrays with a pixel pitch of 16.38 µm and a native fill factor of 10.5%, whereas improved simulation tools could give a better estimate of the actual concentration factor. Spectral measurements were also carried out, resulting in good and uniform transmission in the visible and NIR.

Microlens testing on back-illuminated image sensors for space applications.

The optoelectronic properties of image sensors, among which are the photosensitivity and resolution, are key to the quality factors for imaging as well as spectrometry in Earth observation and scientific space exploration missions. Microlens arrays (MLAs) further improve state-of-the-art CMOS image sensors (CIS) by redirecting more photons into the photosensitive surface/volume of each pixel. This paper reports the design, deposition, optical characterization, and reliability assessment of such an MLA made from a UV-curable hybrid polymer and replicated on a packaged back-illuminated CIS having a pixel pitch of 15.5 µm. We find that such MLAs are highly stable to temperature variations, exposure to humidity, mechanical shocks and vibrations, as well as irradiation by gamma rays, while improving the parasitic light sensitivity by a factor of 1.8. Such MLAs can be applied on a large variety of image sensors, back-illuminated but mostly front-illuminated, with pixel pitches ranging from a few to several hundreds of micrometers, making them suitable for most specifications of the space industry.